1. Field of the invention
This invention relates generally to determining geological properties of subsurface formations using Nuclear Magnetic Resonance (NMR) methods for logging wellbores, particularly for improving the signal level and reducing the power consumption by modifying the pulse sequence timing compared to prior art.
2. Background of the art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
The NMR tools generate a uniform or near uniform static magnetic field in a region of interest surrounding the wellbore. NMR is based on the fact that the nuclei of many elements have angular momentum (spin) and a magnetic moment. The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality. Over time the nuclear spins align themselves along an externally applied magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field, which tips the spins with resonant frequency within the bandwidth of the oscillating magnetic field away from the static field direction. The angle xcex8 through which the spins exactly on resonance are tipped is given by the equation:
xcex8=xcex3B1tpxe2x80x83xe2x80x83(1)
where xcex3 is the gyromagnetic ratio, B1 is the effective field strength of the oscillating field and tp is the duration of the RF pulse.
After tipping, the spins precess around the static field at a particular frequency known as the Larmor frequency xcfx890, given by
xcfx89=xcex3B0xe2x80x83xe2x80x83(2)
where B0 is the static field intensity. At the same time, the spins return to the equilibrium direction (i.e., aligned with the static field) according to an exponential decay time known as the spin-lattice relaxation time or T1. For hydrogen nuclei, xcex3/2xcfx80=4258 Hz/Gauss, so that a static field of 235 Gauss would produce a precession frequency of 1 MHz. T1 of fluid in pores is controlled totally by the molecular environment and is typically ten to one thousand milliseconds in rocks.
At the end of a xcex8=90xc2x0 tipping pulse, spins on resonance are pointed in a common direction perpendicular to the static field, and they precess at the Larmor frequency. However, because of inhomogeneity in the static field due to the constraints on tool shape, imperfect instrumentation, or microscopic material heterogeneities, each nuclear spin precesses at a slightly different rate. Hence, after a time long compared to the precession period, but shorter than T1, the spins will no longer be precessing in phase. This de-phasing occurs with a time constant that is commonly referred to as T2* if it is predominantly due to the static field inhomogeneity of the apparatus, and as T2 if it is due to properties of the material.
The receiving coil is designed so that a voltage is induced by the precessing spins. Only that component of the nuclear magnetization that is precessing in the plane perpendicular to the static field is sensed by the coil. After a 180xc2x0 tipping pulse (or an xe2x80x9cinversion pulsexe2x80x9d), the spins on resonance are aligned opposite to the static field and the xe2x80x9cprecessionxe2x80x9d consists of a slow return along the static field axis to the equilibrium direction. Hence, a signal will be generated after a 90xc2x0 tipping pulse, but not after a 180xc2x0 tipping pulse in a generally uniform magnetic field.
While many different methods for measuring T1 have been developed, a single standard known as the CPMG sequence (Carr-Purcell-Meiboom-Gill) for measuring T2 has evolved. In contrast to laboratory NMR magnets, well logging tools have inhomogeneous magnetic fields due to the constraints on placing the magnets within a tubular tool and the inherent xe2x80x9cinside-outxe2x80x9d geometry. Maxwell""s divergence theorem dictates that there cannot be a region of high homogeneity outside the tool. Therefore in typical well bores, T2*  less than  less than T2, and the free induction decay becomes a measurement of the apparatus-induced inhomogeneities. To measure the true T2 in such situations, it is necessary to cancel the effect of the apparatus-induced inhomogeneities. To accomplish the same, a series of pulses is applied to repeatedly refocus the spin system, cancelling the T2* effects and forming a series of spin echoes. The decay of echo amplitude is a true measure of the decay due to material properties. Furthermore it can be shown that the decay is in fact composed of a number of different decay components forming a T2 spectrum. The echo decay data can be processed to reveal this spectrum which is related to rock pore size distribution and other parameters of interest to the well log analyst.
One method to create a series of spin echoes is due to Carr and Purcell. The pulse sequence starts with a delay of several T1 to allow spins to align themselves along the static magnetic field axis. Then a 90xc2x0 tipping pulse is applied to rotate the spins into the transverse plane where they precess with angular frequency determined by local magnetic field strength. The spin system loses coherence with time constant, T2*. After a short time tCP a 180xc2x0 tipping pulse is applied which continues to rotate the spins, inverting their position in the transverse plane. They continue to precess, but now their phases converge until they momentarily align a further time tCP after the 180xc2x0 pulse. The 180xc2x0 pulse is re-applied after a further time tCP and the process repeated many times forming a series of spin echoes with spacing 2 tCP.
While the Carr-Purcell sequence would appear to provide a solution to eliminating apparatus induced inhomogeneities, it was found by Meiboom and Gill that if the duration of the 180xc2x0 pulses in the Carr-Purcell sequence were even slightly erroneous so that focusing is incomplete, the transverse magnetization would steadily be rotated out of the transverse plane. As a result, substantial errors would enter the T2 determination. Thus, Meiboom and Gill devised a modification to the Carr-Purcell pulse sequence such that after the spins are tipped by 90xc2x0 and start to de-phase, the carrier of the 180xc2x0 pulses is phase shifted by xcfx80/2 radians relative to the carrier of the 90xc2x0 pulse. This phase change causes the spins to rotate about an axis perpendicular to both the static magnetic field axis and the axis of the tipping pulse. If the phase shift between tipping and refocusing pulses deviates slightly from xcfx80/2 then the rotation axis will not be perfectly orthogonal to the static and RF fields, but this has negligible effect. For an explanation, the reader is referred to a detailed account of spin-echo NMR techniques, such as xe2x80x9cNMR: a nuts and bolts approachxe2x80x9d, Fukushima and Roeder. As a result any error that occurs during an even numbered pulse of the CPMG sequence is cancelled out by an opposing error in the odd numbered pulse. The CPMG sequence is therefore tolerant of imperfect spin tip angles. This is especially useful in a well logging tool which has inhomogeneous and imperfectly orthogonal static and pulse-oscillating (RF) magnetic fields,
U.S. Pat. No. 5,023,551 issued to Kleinberg discloses an NMR pulse sequence for use in the borehole environment which combines a modified fast inversion recovery (FIR) pulse sequence with a series of more than ten, and typically hundreds, of CPMG pulses according to
[Wixe2x88x92180xxe2x88x92tixe2x88x9290xxe2x88x92(tcpxe2x88x92180yxe2x88x92tcpxe2x88x92echo)j]ixe2x80x83xe2x80x83(3)
where j=1,2, . . . ,J, and J is the number of echoes collected in a single CPMG sequence, where i=1,2, . . . ,I and I is the number of waiting times used in the pulse sequence, where Wi are the recovery times before the inversion pulse, and where ti are the recovery times before a CPMG sequence, and where tCP is the Carr-Purcell spacing. The phase of the RF pulses 90 and 180 is denoted by the subscripts X and Y, Y being phase shifted by xcfx80/2 radians with respect to X. The subscripts also conventionally relate to the axis about which rotation of the magnetization occurs during the RF pulse in a local Cartesian co-ordinate system centered on the nucleus in which the static magnetic field is aligned in the Z direction and the RF field in the X direction. This sequence can be used to measure both T1 and T2, but is very time consuming, limiting logging speed. If tCP is set to zero and the inverting pulse is omitted then the sequence defaults to standard CPMG for measuring T2 only.
A number of other prior art inventions, such as those disclosed in U.S. Pat. No. 5,363,041 issued to Sezginer, U.S. Pat. No. 5,381,092 issued to Freedman, and International Application WO 97/34167 of Prammer use variations of the CPMG pulse sequence. Fundamental to all of these inventions is the use of a 180xc2x0 refocusing pulse.
The CPMG pulse sequence was developed for use in laboratory and medical settings where the magnets are outside the region of examination and it is relatively easy to maintain a uniform magnetic field strength over the region of examination. In a downhole NMR system, the magnetic field inevitably has a substantial magnetic field gradient compared to a laboratory magnet. This is a direct and unavoidable result of its xe2x80x9cinside-outxe2x80x9d nature: the region of examination is on the outside of the magnets. In order to maximize the NMR signal from such a system, it is necessary to gather the signal from a region as large as possible. To do this, the magnetic field gradients are minimized over a volume of space remote from the tool. However, the remaining magnetic field inhomogeneity is still hundreds to thousands of times larger than in a laboratory magnet. Therefore, the RF pulse bandwidth must be as large as possible to tip nuclei with resonance conditions as far as possible from the resonant frequency, thereby expanding the volume boundaries to the maximum possible.
To achieve a wide bandwidth, the RF pulses must be as short as possible, consistent with the available RF peak power, typically a few kilowatts, which leads to the condition that the 90xc2x0 pulse is shorter than the 180xc2x0 refocusing pulse, and is typically half the duration if the pulses have an approximately square shaped envelope. Consequently, the 90xc2x0 pulse has twice the bandwidth of the 180xc2x0 pulse. As a result of this only about half the nuclei which are tipped by the 90xc2x0 pulses are subsequently refocused by the 180xc2x0 pulse.
The above discussion is an over-simplification because of the substantial inhomogeneity and non-orthogonality of the real static and RF fields, but it captures the essence of the problem of using a CPMG sequence in an inhomogeneous field system.
The present invention is a method of improving the NMR signals received from a formation surrounding a borehole. Any pulsed NMR tool in which a magnet arrangement is used to generate a static magnetic field having a substantially uniform field strength in a region of the formation surrounding the borehole, and in which an RF antenna is used to produce pulsed RF fields substantially orthogonal to the static field in the region of examination may be used. The nuclear spins in the formation align themselves along the externally applied static magnetic field. A pulsed RF field is applied which tips the spins on resonance by the desired tip angle for maximum signal, typically 90xc2x0. A refocusing pulse having a spin tip angle substantially less than 180xc2x0 is applied with carrier phase shifted by typically xcfx80/2 radians with respect to the 90xc2x0 tipping pulse. Although the refocusing pulses result in spin tip angles less than 180xc2x0 throughout the sensitive volume, their RF bandwidth is closer to that of the original 90xc2x0 pulse. Hence more of the nuclei originally tipped by 90xc2x0 are refocused, resulting in larger echoes than would be obtained with a conventional 90xc2x0 refocusing pulse. The reduced duration of the refocusing pulses also reduce the power consumption of the tool.
One embodiment of the invention uses a 90xc2x0 pulse of inverted phase at the end of the sequence to speed up recovery of the longitudinal magnetization by forcing the realignment of the spin system with the static field, and to allow cancellation of the tipping pulse xe2x80x9cringingxe2x80x9d artefact. The NMR echoes are analyzed in a conventional manner to give the NMR parameters of the formation.